Power supplies (PS), in particular switch mode power supplies (SMPS), are designed to convert an electric energy derived from a primary source of electrical power to electrical energy having parameters different from the parameters of the energy from the primary source. The SMPS's may also provide separation of primary and secondary electric circuits. In many cases power supplies are designed to provide energy to a secondary energy receiver, such as a load, where the energy flows just from the primary source to the secondary energy receiver. In various other cases, when the receiver accumulates and/or delivers the energy, the energy may also flow from the receiver to the primary source or from the receiver to another accumulator of energy. In some examples, power supplies may deliver direct current (DC) unipolar voltage, while various power supplies may deliver alternate current (AC) voltage. Various DC power supplies may be capable of reversing polarity of output voltage according to needs.
Regarding DC power supplies it may be appreciated that DC power supplies may provide voltage of different polarity and different direction of energy flow. This situation is depicted in FIG. 1 as current-voltage quadrants (quarters) of operation. By default, when the voltage is positive, energy flows to a receiver, and the power supply operates in the first quadrant (Q-I), which may be characterized by positive voltage and positive current. The majority of known power supplies may operate just in this quadrant. Certain classes of power supplies, for instance, battery chargers or uninterruptible power supplies (UPS), may operate also in a so-called second quadrant (Q-II), where the voltage does not change polarity, while current (which may be termed negative current) flows in the reverse direction (battery is charging or discharging). The same situation may take place when a receiver is a unidirectional DC motor, which can return the energy to the primary source or intermittent energy accumulator. The bidirectional DC motor requires bidirectional voltage and can also return the energy. Thus, operation may take place in all four quadrants as shown in FIG. 1. By convention the quadrant where the voltage is negative and power flows to the receiver is called the third quadrant (Q-III), while the fourth quadrant is the quadrant where the voltage is negative and energy flows back from the receiver (Q-IV).
In view of the above, power supplies may be divided into three classes: a first class, single quadrant power supply, operating in one quadrant: Q-I or Q-III; a second class, two-quadrant power supply, operating in two quadrants Q-I and Q-II or Q-III and Q-IV; and a third class, four quadrant power supply, operating in all four quadrants. Notably, in the class of the two-quadrant power supplies, power supplies do not generally operate in Q-I and Q-IV or Q-III and Q-II, where the voltage polarity switches while the direction of current flow does not.
In the majority of welding power supplies that are DC one-quadrant power supplies, operating in Q-I or Q-III, the DC welding process requires just delivery of the energy in a controlled manner, in particular with fast changes. The output current in this process does not reverse or need to reverse its direction. However, during DC welding, the energy in the receiver is not just consumed in the welding processed, but also may accumulate in an output inductor of the power supply and in the supplying cables, which physically act as inductors. According to the principle of electromagnetic induction, the current in the inductor does not change immediately. The derivative of the current in the inductor in time (dI/dt) is proportional the voltage applied to the inductor. Therefore, for fast control of the output current it would be reasonable to reverse the output voltage of the power supply. A one-quadrant power supply can provide just positive voltage. Thus, during short circuit conditions the voltage applied to the inductors may just be positive or a very slightly negative. Consequently, while the current may increase very rapidly, the current decreases just very slowly. This circumstance represents a major drawback of a one-quarter power supply.
An example of a one-quadrant power supply 200 during the short circuit is presented in FIG. 2. The converter employs primary side full bridge (FB) formed from switches VT1, VT2, VT3, VT4 with respected reverse diodes VD1, VD2, VD3, VD4. In the example of FIG. 2, the energy flows just in one direction—from mains through the rectifier, which is schematically shown by the diode VD0. The DC bus capacitor C1 provides bidirectional conductivity, which in this case is needed just to discharge leakage and magnetizing inductances of the power transformer T1. The converter employs a center tap active rectifier (CTAP) with diodes VD5 and VD6. The output current is smoothed by means of the output inductor L2 and inductances of the output cables. The output inductor, inductances and resistance of cables and the welding load constitutes an energy receiver. As mentioned previously, because of the nature of welding, during short circuits fast control of the output current is needed. While the current can increase very fast, a fast current decrease is not achievable. When fast current decrease is commanded, all the primary switches VT1, VT2, VT3, VT4 must be off. The diode VD5 and diode VD6 are conducting in a free-wheeling manner. The voltage applied to the output inductor L2 and the inductances of the cable Lcable+, Lcable− is small, and equal to the sum of the voltage drops on the cables and the output rectifier. A similar, but not so severe situation, occurs during pulse welding. High rates of dI/dt, especially during the decreasing phase are expected; however the voltage applied over the inductances is limited due to the one-quadrant operation of the power supply.
In view of the above, known welding power supplies have been designed for forcing a reduction of current during a short circuit using a switch connected in series in the output circuit, as shown in FIG. 3 and FIG. 4, where the secondary side of a power supply is shown. In the power supply 300 of FIG. 3 or the power supply 400 of FIG. 4, under normal operation, the switch VT7 is closed (turned on) and the respective power supply delivers power and current to the welding pool. During a short circuit and when the process control requires a rapid decrease of current, the switch VT7 is opened (turned off), and the output current I2 flows through the resistor R2 (FIG. 3) or through a voltage clamp depicted as an equivalent power Zener diode, VT10 (see FIG. 4). The large reverse voltage is applied to the inductors (output inductor and cables), causing a rapid decrease of the current. In the two power supplies, the energy accumulated in the inductances is dissipated, either by the resistor R2 or by the voltage clamp (VT10), which processes may be very ineffective. In addition, terminating current of levels 300 A to 400 A may cause big voltage surges and may require utilization of ineffective high voltage devices and snubbers.
With respect to these and other considerations, the present disclosure is provided.